The elastic modulus of spring steel is usually quoted at somewhere between 200 and 210GPa, so 205GPa was used in the original Wyatt spreadsheet as a typical value. I don't think anyone is going to be worried about a 2% difference in deflection value, and besides which no one really knows what the value is for a bit of plated Ernie Ball E string.

The CSB Source Book post (the first in this thread) edited on 31/3/2104 to reference a new summary of the “Abuse CSB Theory” thread. This summary is the post that follows this one but only makes complete sense if read via the link in the CSB Source Book post.

CSB source book summary of the Abstruse CSB Theory Thread, which is for people who are actually interested in the way the spreadsheets used to sort out CSB fulcrum placement go about their business. There is no doubt this is a minority interest and can tend to confuse. Therefore, for those just wanting to know where to put their CSB fulcrums, this is distinctly NOT compulsory reading.

The thread starts with 4 posts which bring out the fact that, while it is simple to calculate where a locos Centre of Gravity (CofG) is from the weight carried on each axle, for a sprung chassis you cannot easily reverse the calculation. That is, you can't easily work out from actual location of the CofG what weight will be carried by each axle. To do so requires that you know a great deal about the characteristics of the springs. Much of the discussion that follows stems from this simple truth.

This starts with a post from John Bateson concerning his calculation of a set of CSB fulcrum points which produced a highly volatile solution, not like the robust outcomes I was claiming. This is followed by a series of posts that discussed the fact that there is always more than one possible solution and that he had chosen an extreme one. 3 more stable possibilities are provided. Each solution has slightly different characteristics. One post contains a diagram which I still find useful to help visualise these differences, and determine which solution to choose.

The latter two thirds of the thread is taken up with a discussion of the numerical methods used by the spread sheets to calculate fulcrum positions. The original Roger Wyatt spread sheet goes about it in a way that isn’t immediately intuitive, and a lot of the discussion here was around determining if it was in fact valid. This led to Alan Turner writing a new spread sheet, using a totally different method, which magically turns out to give the same answers. That is at least when considering the fulcrum point placement, which was the whole point after all. There is still stuff about the weight distribution which we will eventually come back to.

In defence of the CSB Spread SheetThis was the only set piece posting in the midst of the discussion, in which I finally managed to explain the circumstances in which the Roger Wyatt methods was an applicable and effective tool. I'm not sure Alan was ever fully convinced.

The practical upshot is that we now have two different methods for approaching the same job, implemented as spreadsheets. These have different characteristics giving you a choice of which one you feel most comfortable with. The results are the same. While copies of Alan's spread sheet are contained within the thread, anybody wanting a copy should download the most recent versions from theCLAG website.

I downloaded the CLAG spreadsheet, the spread sheet on here and the simplified 3 axel spread sheet. If I insert the figures calculated by the auto calculate feature into the other two spread sheets (the fulcrum points, load weights and wire thickness) the other two agree but do not match the figures in the auto calculations for the deflections? Am I doing something wrong?

Le Corbusier wrote:I downloaded the CLAG spreadsheet, the spread sheet on here and the simplified 3 axel spread sheet. If I insert the figures calculated by the auto calculate feature into the other two spread sheets (the fulcrum points, load weights and wire thickness) the other two agree but do not match the figures in the auto calculations for the deflections? Am I doing something wrong?

Tim

Probably not. All these spreadsheets use the same calculation as contained in the basic Roger Wyatt spread sheet. What is different is that the version of Rogers Spreadsheet on the CLAG website has been updates, mostly cosmetically, at some point and there has been a minor change in the value of the Elastic Modules of steel it uses. This has bee modified from 2.00E+05 to 2.05E+5. The original value continues to be used in the other spread sheets. If you make this value the same in all the spread sheets you will get the same answers.

Modifying the elastic modules has the same effect as changing (marginally) the size of the spring wire. That is, it affects the absolute size of the deflection figures, but not the relative size of the the defection across all the axles, which is what matters.

Which value is more accurate I'm not sure, however the whole effect is significantly less then the effect of changing the wire sizes by 1 thou either way so the only significant effect would be to worry somebody who is comparing the results between the two.